In muscular contraction, electrical nerve impulses or action potentials are conducted along nerve pathways to muscle fibers at the myoneural junction. Muscle fibers, thus excited, contract as the action potential instantaneously propagates along the fibers. For a single muscle fiber, this event is known as a motor unit action potential. In muscular contraction, gross muscular tension is effected by a series of excitations through many muscle fibers.
When many muscle fibers contract simultaneously, the total electrical potential arising from the conductive summation of individual motor unit action potentials measured at the skin is great enough to be electronically measured with an electromyogram (EMG). Instantaneous EMG measurement is accomplished by attaching surface electrodes to the skin or inserting needle electrodes into the muscle and recording the instantaneous changes in the magnitude of the raw myographic signal. In integrated EMG measurement (IEMG), the raw signal is integrated, or time averaged, over a short period of time and then recorded.
This latter method is preferential in sensory feedback motor training, wherein the overall magnitude of the myographic response is of primary importance. Changes in the level of response by a conscious effort to contract or relax muscle groups on the part of the subject is "fed back" to the subject in the form of sensory stimuli. Invariably, the stimulus is either auditory, visual or a combination of the two, and its strength usually is proportional to the level of motor activity. Thus, where the pathways of neuromuscular communication and control are weak, damaged, decerebrate, untrained or lacking in proprioceptive effect, the feedback stimulus provides a singular method for bringing muscular coordination into the arena of conscious awareness and control. While the mechanisms of sensory feedback are not as yet precisely known, the technique is found to produce positive results in many areas of clinical, physical, sports and behavioral medicine.
Electrical stimulation of excitable nerve and muscle tissue is an important therapeutic modality applicable to the clinical treatment of neuromuscular and muscoloskeletal problems wherein the tension of skelatal musculature is affected through gross elicitation of motor unit contraction by an externally applied electrical current. Electrical muscle stimulation (EMS) devices typically employ time varying waveforms which are applied to specific surface sites on muscle groups. Clinical application of electrical stimulation includes facilitation of voluntary motor function, muscle strength enhancement, motion range improvement and spasticity inhibition. Reference is made to Benton et al, "Functional Electrical Stimulation--A Practical Clinical Guide", Second Edition, Ranchos Los Amigos Rehabilitation Engineering Center, Downey, Calif., pages 31-52.
It is thus clear that sensory feedback and EMS share many of the same patient populations and treatment objectives. Patients suffering central nervous system insult, either from head trauma or stroke, and spinal injury patients having incomplete damage to the spinal cord and orthopedic patients, may be treated by rehabilitation programs involving EMS and sensory feedback administered separately.
Recently, work has been undertaken to combine EMS and EMG technology to produce bioelectrically controlled electrical stimulators incorporating a feedback system to provide information on the correspondence between a preset program of movements and motions actually performed by a subject. In U.S. application Ser. No. 697,897, filed on Feb. 4, 1985 now abandoned and assigned to the assignee of this invention, EMG measurements made near the muscles to be electrically stimulated are compared with an EMG signal threshold level corresponding to a programmed therapeutic goal. When the measured EMG signal meets or exceeds this EMG threshold level, electrical muscle stimulation is applied to the muscle site, to effect kinesthetic movement of the muscle group from which the EMG signal is being measured. EMS stimulation is, however, applied with a "zero-bang" control strategy, that is, once the stimulation cycle has been voluntarily activated on the part of the subject, by virtue of the EMG magnitude, motor activation is realized purely as a function of the preprogrammed EMS cycle, rather than volitional control. However, we now believe that significant improvements in neuromuscular re-education can be realized by volitional control electrical muscle stimulation, not provided heretofore. We further believe that application of volitional control electrical muscle stimulation incorporating the technologies of EMG and EMS therapy in a unique manner may produce syneristic and accelerated therapeutic benefits to the neurologically compromised subject beyond those of traditional EMG and EMS treatment. This is in contrast with the current clinical use of EMS wherein control of muscles by electrical stimulation is passive, i.e., not controlled as a function of neurological signals developed by the subject.
It is accordingly an object of the invention to provide a method and system for improved neuromuscular reeducation.
It is a more specific object to provide a method of and system for volitional control electrical muscle stimulation, i.e., muscle stimulation by an electrical signal that is a predetermined, preferably linear, function of the IEMG signal.
During our experimentation with a human subject undergoing neuromuscular rehabilitation therapy involving application of EMS signals to a muscle site and making EMG measurements in close proximity to or specifically at that site, the EMG measurement tends to become obscured by an EMG artifact that follows each EMS pulse applied to the muscle. In those experiments, we have found that the EMG signal in fact becomes unusable as a therapeutic parameter at high levels of EMS, particularly when EMG and EMS take place at a common site. It is accordingly an additional object of the invention to provide a system for "masking" obscuration of the raw EMG signal by EMS pulses applied to the muscles in therapy involving volitional control electrical muscle stimulation.